Mountain Bikers of the Central Okanagan
This is a very active hard working group of mountain bikers! The website is a terrific resource for the area. Lots of information on trails, group rides and MTB events!
Friends of the South Slopes Society (FOSS)
This is a volunteer run organization that does a lot of trail work that benefit mountain bikers in Myra-Bellevue & Okanagan Mountain Provincial Parks and other public lands on the South Slopes. This is a very large area that is well maintained and provides trail networks for most skill levels. Great views of Kelowna and Lake Okanagan.
I want to add this link. As you are probably aware of this website, it is a great resource for trail maps and information in this and most other areas.